feat: add ai-gateway-cost-control experiment flag (#26399)

Adds the `ai-gateway-cost-control` experiment flag to gate new cost
control endpoints and upcoming frontend UI behind an explicit opt-in.

Currently AI Gateway cost control supports the following endpoints:
- `GET/PUT/DELETE /api/v2/organizations/{org}/groups/{group}/ai/budget`
- `GET/PUT/DELETE /api/v2/users/{user}/ai/budget`

Note: the group-level endpoints were already released in v2.34.0 and
remain ungated. Only the user-level endpoints are gated behind this
experiment. Future cost control endpoints and UI should use this
experiment for gating until the feature is stable.

> Generated by Coder Agents on behalf of @ssncferreira

docs/reference/api/schemas.md

@@ -6961,9 +6961,9 @@ CreateWorkspaceRequest provides options for creating a new workspace. Only one o
 
 #### Enumerated Values
 
-| Value(s)                                                                                                                                                                |
-|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| `auto-fill-parameters`, `example`, `mcp-server-http`, `minimum-implicit-member`, `nats_pubsub`, `notifications`, `oauth2`, `workspace-build-updates`, `workspace-usage` |
+| Value(s)                                                                                                                                                                                           |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `ai-gateway-cost-control`, `auto-fill-parameters`, `example`, `mcp-server-http`, `minimum-implicit-member`, `nats_pubsub`, `notifications`, `oauth2`, `workspace-build-updates`, `workspace-usage` |
 
 ## codersdk.ExternalAPIKeyScopes